1. Field of the Invention
The present invention generally relates to an analog-to-digital converter circuit and a signal processing circuit, and it particularly relates to a pipeline type analog-to-digital converter circuit and a signal processing circuit using said pipeline-type analog-to-digital converter circuit.
2. Description of the Related Art
As a CCD (Charge-Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor) sensor have been mounted on portable equipment such as mobile phones, the number of pixels thereof exceeds million pixels and tends to rise continuously. Under such circumstances, both reduced power consumption in the processing and higher precision are demanded for analog front-end (AFE) circuits that process output signals from the image sensors.
As a conventional AFE circuit for use in image sensors, there is a circuit equipped with a function wherein in order to enhance color gradation the gain is varied according to the amplitude of an input signal from the image sensor and an input signal is amplified. That is, when the level of the input signal is small during a certain period of time, this AFE circuit adjusts (amplifies) the signal amplitude thereof by raising the gain and thereby the resolution of a subsequent analog-to-digital (A-D) converter circuit is raised.
In the aforementioned AFE circuit, a variable amplifier circuit for amplifying analog signals needs to be provided before the A-D conversion. However, a problem arises where the variable amplifier circuit may generate noise that degrades the image quality.